Auribromic Acid, HAuBr₄

Solutions of auric bromide and of gold in bromine-generating liquids are converted by hydrogen bromide into a solution of auribromic acid. The substance is also formed in solution by the action of the same reagent on a solution of auric chloride, and can be extracted by ether from the dark-red liquid. Evaporation of the concentrated aqueous solution yields the acid in dark-red crystals of the formula HAuBr₄,5H₂O or HAuBr₄,6H₂O, melting at 27° C. in its own water of crystallization, and decomposed by concentrated sulphuric acid at 155° C. into aurous bromide and bromine.

Potassium auribromide, KAuBr₄, is produced by the action of bromine on gold and the equivalent proportion of potassium bromide. On evaporation of the solution, potassium auribromide crystallizes in dark-red prisms containing two molecules of water of crystallization; and by drying the crystals over phosphoric oxide it is obtained in the form of purple-red, monoclinic crystals free from water. On exposure to air, it takes up two molecules of water. Its solubility at 15° C. is 19.53 grams in 100 grams of water.

Ammonium auribromide, NH₄AuBr₄, is produced by the action of ammonia on the acid, and forms bluish-black crystals of fat-like lustre. The anhydrous rubidium and caesium salts give prisms of metallic lustre, the colour of the first being cinnabar-red, and of the second deep black.

Auribromides of sodium, barium, zinc, manganese, and magnesium have been prepared. Triple bromides of gold, silver, and rubidium (or caesium) have also been described. Double compounds of auric bromide and phosphorus derivatives can be obtained by methods analogous to those employed for the corresponding chlorine addition-products.